Recently, many linearized amplifiers have come to be used in base station radio equipment for mobile communication systems to reduce the sizes of base stations.
FIG. 32 is a block diagram showing a feedforward amplifier as a typical example of linearized amplifiers. The feedforward amplifier shown in FIG. 32 includes delay circuits 321, directional couplers 322, 323, and 325, a main amplifier 324, an error amplifier 326, an input terminal 327, and an output terminal 328. Main signals are input from the input terminal 327 and are amplified in the main amplifier 324. In the signals amplified in the main amplifier 324, distortion occurs and only distorted components are detected in a carrier cancellation loop. The feedforward amplifier is a circuit in which only the distorted components are eliminated from the signals including the distortion, which has been amplified in the main amplifier 324, in the distortion cancellation loop, and only signals including no distortion are extracted. The details of its operation are described in “High-Power GaAs FET Amplifiers” by John L. B. Walker (issued by Artech House (Boston, London), see 7.3.2 Linearized Amplifiers). In the carrier cancellation loop and the distortion cancellation loop, in order to allow the group delay times of the two signals divided in the directional coupler 323 to coincide exactly with each other and to synthesize them in the directional coupler 325, strict and fine adjustment of the group delay times is required for the delay circuits 321.
Conventionally, in a distortion compensating circuit in a linearized amplifier, for the purpose of adjusting group delay times, a delay device using a coaxial cable such as one with a diameter of about 2 cm and a length of at least 10 m has been used in general.
However, such a delay device is large and has a great insertion loss, which have been disadvantages. The great insertion loss requires the device to have a higher output power, thus causing various problems such as an increase in the size of equipment, a high power consumption, a further complicated configuration relating to radiation, or the like, which have been obstacles to obtaining small base station equipment. Furthermore, it is required to vary the physical length of a cable for carrying out the fine adjustment of the group delay time. Therefore, each time the length is varied, it is necessary to disconnect connectors and to cut the cable, resulting in a poor working efficiency, which has been a problem.
On the other hand, a dielectric filter mainly has been used for removing undesired signals as a bandpass filter or a band stop filter, and particularly, its amplitude transfer characteristics have received attention. Therefore, conventional dielectric filters have low losses, but a deviation in group delay time depending on frequencies is great. For this reason, it has been considered that the conventional dielectric filters cannot be used for delay devices providing uniform group delays. Moreover, it has been hardly intended to flatten both amplitude characteristics and group delay frequency characteristics at the same time. In addition, there has been no example of achieving both the low loss and the reduction in size using a dielectric.